if you want to use bmp image created using photoshop, you have to do like this:

create the image with photoshop and after you finish merge all the visible layer and change the image mode to indexed color and then save as bmp. After saving open the image with irfanview in my case. Then in the irfanview click on the image tab and then go to information in the information menu change the resolution as the resolution you have selected in your ezboot option. If your seleted resolution in ezboot option is 800x600 change the resolution of your image to 800x600 from the irfanview then save again as bmp. Now you can use that image.

I was making my images in photoshop then saving them. Then I would open my saved image (doesn't matter what format) in Microsoft Paint to save it as a 256 color bitmap. We have alot of machines that do not support more than 256 colors unfortunately. But they would come out nearly perfect all the time.